A semiconductor device typically includes a network of circuits that are formed over a substrate. The device may consist of several layers of circuit wiring, with various interconnects being used to connect these layers to each other and any underlying transistors. Generally, as a part of the manufacturing process, vias or contact holes (hereafter, collectively referred to as vias) are formed, which are transferred to another layer and then filled with a metal to form interconnects, so that the various layers of circuitry are in electrical communication with each other. Prior art methods of forming interconnects generally rely on a series of lithographic and etching steps to define the positions and dimensions of the vias, which in turn define the positions and dimensions of the corresponding interconnects. To this end, photoresists and hard masks may be employed. However, the dimensions of features formed using conventional optical lithography techniques for volume manufacturing (e.g., 193 nm dry and immersion lithography) have reached the resolution limit of the lithographic tools.
The creation of vias with smaller critical dimensions (CDs), tighter pitches, and better CD uniformity is one of major challenges for future technology nodes; however, printing such via patterns beyond the 22 nm node is expected to be difficult using conventional optical lithography, even with expensive and complicated double patterning processes, resolution enhancement technology (computational lithography) and severe layout design restrictions. Unfortunately, no alternative non-optical lithographic technique with higher resolution capabilities, such as e-beam lithography or extreme ultraviolet lithography (EUV), appears to be ready for high volume manufacturing in the near future. While e-beam direct write lithography is capable of very high resolution, it is a direct-write technique and cannot achieve the necessary wafer throughput levels to make it viable for volume manufacturing. EUV lithography tools have been under development for many years; however, many challenges associated with the source, collection optics, masks, and resists still remain and will likely delay any practical implementation of EUV lithography for several years.
Block copolymer (BCP) patterning has attracted attention as a possible solution to the problem of creating patterns with smaller dimensions. Under the right conditions, the blocks of such copolymers phase separate into microdomains (also known as “microphase-separated domains” or “domains”) to reduce the total free energy, and in the process, nanoscale features of dissimilar chemical composition are formed. The ability of block copolymers to form such features recommends their use in nanopatterning, and to the extent that features with smaller CDs can be formed, this should enable the construction of features which would otherwise be difficult to print using conventional lithography. However, without any guidance from the substrate, the microdomains in a self-assembled block copolymer thin film are typically not spatially registered or aligned.
To address the problem of spatial registration and alignment, directed self-assembly (DSA) has been used. This is a method that combines aspects of self-assembly with a lithographically defined substrate to control the spatial arrangement of certain self-assembled BCP domains. One DSA technique is graphoepitaxy, in which self-assembly is guided by topographical features of lithographically pre-patterned substrates. BCP graphoepitaxy provides sub-lithographic, self-assembled features having a smaller characteristic dimension than that of the prepattern itself.
Some initial applications of DSA based on BCP graphoepitaxy have been reported. Directed self-assembly of block copolymers has been used to reduce the diameter of holes created with conventional lithographic methods, as illustrated in FIG. 1 (see, for example, US Published Patent Application 20080093743A1). With this technique, a solution containing a block copolymer is applied on a topographical substrate 120 having openings 124 therein (FIG. 1A), thereby filling the openings. (For the sake of clarity, only a portion of the substrate is shown in each of the figures herein.) Microphase-separated domains 128 and 132 are then formed in the openings 124 (FIG. 1B) as a result of an annealing process. The discrete, segregated polymer domains 132 formed in the centers of the openings 124 are subsequently removed via an etch process to create holes 136 that are smaller than the corresponding openings 124. Note, however, that the pitch of the pattern realized with this approach is unchanged from the pitch of the starting lithographic pre-pattern (i.e., there is no increase in pattern density).
Overall pattern density (related here to the smaller CD and smaller pitch) has been increased by creating an array of self-assembled polymer domains in a lithographically defined trench 140, as shown in FIG. 2A (see Cheng et. al., Applied Physics Letters, 2002, 81, 3657). However, there was effectively no control of the placement of each self-assembled domain 144 (FIG. 2B), and hence no control over the final location of the corresponding holes 148 generated as a result of the etch process (FIG. 2C). Thus, these holes 148 do not form an array in which the domains have predetermined positions, and the standard deviation of these positions can vary from a precise array by as much as 10% of the average center-to-center domain spacing (see Cheng et. al., Advanced Materials 2006, 18, 2505). A variation of this magnitude makes such a directed self-assembled method unsuitable for patterning devices requiring a standard deviation σ in placement of 3.5% (3σ˜10%) of CD.
As shown in FIG. 3A, one or more widely separated indentations 160 in the sidewall of a prepatterned trench (made by e-beam lithography) have been incorporated in an attempt to register hexagonal arrays of block copolymer domains (see C. Cheng et. al., Advanced Materials 2003, 15, 1599; and Cheng et. al. Nature Materials 2004, 3, 823). However, the indentations 160 did not exert enough influence to achieve the desired positional accuracy of the domains 164 (and thus of their corresponding holes 168), nor did they break the hexagonal symmetry of the corresponding array.